The present invention relates to a liquefaction of diphenylmethane diisocyanate (hereinafter referred to as "MDI") and a process for preparing liquid MDI.
MDI has a very high reactivity and is a very useful compound as a raw material for polyurethane resin prepared by the reaction with a polyol mixture containing an active hydrogen compound, and an additive such as a chain extender and a catalyst.
Polyurethanes prepared from MDI are widely used as molded articles such as bumpers of automobile, foams such as seat cushions, cast elastomers, sealing materials and the like.
MDI usually comprises the 4,4'-isomer together with a small amount of the 2,4'-isomer. Both isomers are solid at room temperature and their melting points are 42.degree. C. and 36.degree. C., respectively. MDI often contains small amounts of the 2,2'-isomer. In any event, MDI, which is a mixture containing 4,4'-isomer, is solid at room temperature.
When MDI is used for a raw material for polyurethane, it is advantageous that MDI be liquid, in view of the storing and handling of the raw material. For this purpose, there is a method for preparing liquid MDI in which a catalyst is added to MDI to conduct a reaction (Japanese Patent Kokoku Publication No. 7545/1970, corresponding to U.S. Pat. No. 3,384,653). However, this method is not always a sufficient method for preparing liquid MDI, since MDI considerably discolors because of a relatively large amount of catalyst and a high reaction temperature and the removal of a solid by-product is necessary.
Japanese Patent Kokai Publication No. 100600/1977 (corresponding to U.S. Pat. No. 4,120,884) discloses a method for preparing liquid having a good storage stability, which comprises adding a phospholine catalyst to MDI to produce a carbodiimide. This method is preferable, since the amount of the added catalyst is small, e.g., several ppm to several dozen ppm. The phospholine catalyst has a very high activity so that the amount of the added catalyst is very small, but an impurity such as an acid or hydrolyzable chlorine, which is contained in a very small amount in MDI, gives a strong effect on MDI and the same amounts of the added catalyst do not necessarily give liquid MDI having stable properties.
The above problems become even more pronounced when the composition of the isomers of MDI used for liquefaction is varied and the change of content of the small amount impurity increases. In some cases, the addition of catalyst can give no carbodiimidization so that MDI having the desired properties cannot be obtained.